AD8224ACPZ-R7 datasheet(24/28 Pages) AD

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AD8224

Rev. B | Page 24 of 28

APPLICATIONS INFORMATION

DRIVING AN ADC

An instrumentation amplifier is often used in front of an ADC

to provide CMRR and additional conditioning such as a voltage

level shift and gain (see Figure 62). In this example, a 2.7 nF

capacitor and a 500 Ω resistor create an antialiasing filter for the

AD7685. The 2.7 nF capacitor also serves to store and deliver

the necessary charge to the switched capacitor input

of the ADC. The 500 Ω series resistor reduces the burden

of the 2.7 nF load from the amplifier. However, large source

impedance in front of the ADC can degrade the total harmonic

distortion (THD).

For applications where THD performance is critical, the series

resistor needs to be small. At worst, a small series resistor can

load the AD8224, potentially causing the output to overshoot

or ring. In such cases, a buffer amplifier, such as the AD8615

should be used after the AD8224 to drive the ADC.

AD8224

AD7685

4.7µF

ADR435

+5V

2.7nF

REF

500Ω

1.07kΩ

+2.5V

+IN

–IN

±50mV

+5V

0.1µF

10µF

Figure 62. Driving an ADC in a Low Frequency Application

DIFFERENTIAL OUTPUT

The differential configuration of the AD8224 has the same

excellent dc precision specifications as the single-ended output

configuration and is recommended for applications in the

frequency range of dc to 1 MHz.

The circuit configuration, outlined in Table 4 and Table 7, refers

to the configuration shown in Figure 63 only. The circuit includes

an RC filter that maintains the stability of the loop.

The transfer function for the differential output is

VDIFF_OUT

= V+OUT − V−OUT = (V+IN − V−IN) × G

where:

kΩ

49.4

1 +

+IN

–IN

–

AD8224

+OUT

33pF

–OUT

+IN2

REF2

20kΩ

Figure 63. Differential Circuit Schematic

Setting the Common-Mode Voltage

The output common-mode voltage is set by the average of +IN2

and REF2. The transfer function is

VCM_OUT

= (V+OUT + V−OUT)/2 = (V+IN2 + VREF2)/2

+IN2 and REF2 have different properties that allow the

reference voltage to be easily set for a wide variety of applications.

+IN2 has high impedance but cannot swing to the positive

supply rail. REF2 must be driven with a low impedance but

can go 300 mV beyond the supply rails.

A common application sets the common-mode output voltage

to the midscale of a differential ADC. In this case, the ADC

reference voltage is sent to the +IN2 terminal, and ground is

connected to the REF2 terminal. This produces a common-

mode output voltage of half the ADC reference voltage.

2-Channel Differential Output Using a Dual Op Amp

Another differential output topology is shown in Figure 64.

Instead of a second in-amp, ½ of a dual OP2177 op amp creates

the inverted output. Because the OP2177 comes in an MSOP,

this configuration allows the creation of a dual-channel,

precision differential output in-amp with little board area.

Errors from the op amp are common to both outputs and are,

thus, common mode. Errors from mismatched resistors also

create a common-mode dc offset. Because these errors are

common mode, they are likely to be rejected by the next

device in the signal chain.

+IN

–IN

REF

AD8224

VREF

4.99kΩ

–

OP2177

+OUT

–OUT

4.99kΩ

Figure 64. Differential Output Using Op Amp

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